Three-Interface Firewall
Setting up a Linux system as a firewall for a small network with DMZ is a
fairly straight-forward task if you understand the basics and follow the
documentation.
This guide doesn't attempt to acquaint you with all of the features of
Shorewall. It rather focuses on what is required to configure Shorewall in
one of its more popular configurations:
Linux system used as a firewall/router for a small local network.
Single public IP address. If you have more than one public IP address,
this is not the guide you want -- see the Shorewall Setup Guide instead.
DMZ connected to a separate ethernet interface.
Connection through DSL, Cable Modem, ISDN, Frame Relay, dial-up, ...
Here is a schematic of a typical installation.
Shorewall requires that you have the iproute/iproute2 package installed
(on RedHat, the package is called iproute). You can tell if this package
is installed by the presence of an ip program on your firewall system. As
root, you can use the 'which' command to check for this program:
[root@gateway root]# which ip /sbin/ip [root@gateway root]#
I recommend that you first read through the guide to familiarize yourself
with what's involved then go back through it again making your
configuration changes. Points at which configuration changes are
recommended are flagged with . Configuration notes that are unique to
LEAF/Bering are marked with
If you edit your configuration files on a Windows system, you must
save them as Unix files if your editor supports that option or you must
run them through dos2unix before trying to use them. Similarly, if you
copy a configuration file from your Windows hard drive to a floppy disk,
you must run dos2unix against the copy before using it with Shorewall.
Windows Version of dos2unix
Linux Version of dos2unix
PPTP/ADSL
If you have an ADSL Modem and you use PPTP to communicate with a
server in that modem, you must make the changes recommended here in
addition to those detailed below. ADSL with PPTP is most commonly found
in Europe, notably in Austria.
Shorewall Concepts
The configuration files for Shorewall are contained in the directory
/etc/shorewall -- for simple setups, you will only need to deal with a few
of these as described in this guide. After you have installed Shorewall,
download the three-interface sample, un-tar it (tar -zxvf
three-interfaces.tgz) and and copy the files to /etc/shorewall (the files
will replace files with the same names that were placed in /etc/shorewall
when Shorewall was installed).
As each file is introduced, I suggest that you look through the actual
file on your system -- each file contains detailed configuration
instructions and default entries.
Shorewall views the network where it is running as being composed of a set
of zones. In the three-interface sample configuration, the following zone
names are used:
Name Description
net The Internet
loc Your Local Network
dmz Demilitarized Zone
Zone names are defined in /etc/shorewall/zones.
Shorewall also recognizes the firewall system as its own zone - by
default, the firewall itself is known as fw.
Rules about what traffic to allow and what traffic to deny are expressed
in terms of zones.
You express your default policy for connections from one zone to another
zone in the /etc/shorewall/policy file.
You define exceptions to those default policies in the
/etc/shorewall/rules file.
For each connection request entering the firewall, the request is first
checked against the /etc/shorewall/rules file. If no rule in that file
matches the connection request then the first policy in
/etc/shorewall/policy that matches the request is applied. If that policy
is REJECT or DROP the request is first checked against the rules in
/etc/shorewall/common if that file exists; otherwise the file
/etc/shorewall/common.def is checked
The /etc/shorewall/policy file included with the three-interface sample
has the following policies:
Source Zone Destination Zone Policy Log Level Limit:Burst
loc net ACCEPT
net all DROP info
all all REJECT info
In the three-interface sample, the line below is included but commented
out. If you want your firewall system to have full access to servers on
the internet, uncomment that line.
Source Zone Destination Zone Policy Log Level Limit:Burst
fw net ACCEPT
The above policy will:
allow all connection requests from your local network to the internet
drop (ignore) all connection requests from the internet to your firewall
or local network
optionally accept all connection requests from the firewall to the
internet (if you uncomment the additional policy)
reject all other connection requests.
At this point, edit your /etc/shorewall/policy file and make any
changes that you wish.
Network Interfaces
The firewall has three network interfaces. Where Internet connectivity is
through a cable or DSL "Modem", the External Interface will be the
ethernet adapter that is connected to that "Modem" (e.g., eth0) unless
you connect via Point-to-Point Protocol over Ethernet (PPPoE) or
Point-to-Point Tunneling Protocol (PPTP) in which case the External
Interface will be a ppp interface (e.g., ppp0). If you connect via a
regular modem, your External Interface will also be ppp0. If you connect
using ISDN, you external interface will be ippp0.
If your external interface is ppp0 or ippp0 then you will want to set
CLAMPMSS=yes in /etc/shorewall/shorewall.conf.
Your Local Interface will be an ethernet adapter (eth0, eth1 or eth2) and
will be connected to a hub or switch. Your local computers will be
connected to the same switch (note: If you have only a single local
system, you can connect the firewall directly to the computer using a
cross-over cable).
Your DMZ Interface will also be an ethernet adapter (eth0, eth1 or eth2)
and will be connected to a hub or switch. Your DMZ computers will be
connected to the same switch (note: If you have only a single DMZ system,
you can connect the firewall directly to the computer using a cross-over
cable).
Do not connect the internal and external interface to the same hub or
switch except for testing AND you are running Shorewall version 1.4.7 or
later. When using these recent versions, you can test using this kind of
configuration if you specify the arp_filter option in
/etc/shorewall/interfaces for all interfaces connected to the common
hub/switch. Using such a setup with a production firewall is strongly
recommended against.
The Shorewall three-interface sample configuration assumes that the
external interface is eth0, the local interface is eth1 and the DMZ
interface is eth2. If your configuration is different, you will have to
modify the sample /etc/shorewall/interfaces file accordingly. While you
are there, you may wish to review the list of options that are specified
for the interfaces. Some hints:
If your external interface is ppp0 or ippp0, you can replace the "detect"
in the second column with "-".
If your external interface is ppp0 or ippp0 or if you have a static IP
address, you can remove "dhcp" from the option list.
IP Addresses
Before going further, we should say a few words about Internet Protocol
(IP) addresses. Normally, your ISP will assign you a single Public IP
address. This address may be assigned via the Dynamic Host Configuration
Protocol (DHCP) or as part of establishing your connection when you dial
in (standard modem) or establish your PPP connection. In rare cases, your
ISP may assign you a static IP address; that means that you configure your
firewall's external interface to use that address permanently. Regardless
of how the address is assigned, it will be shared by all of your systems
when you access the Internet. You will have to assign your own addresses
for your internal network (the local and DMZ Interfaces on your firewall
plus your other computers). RFC 1918 reserves several Private IP address
ranges for this purpose:
10.0.0.0 - 10.255.255.255 172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255
Before starting Shorewall, you should look at the IP address of your
external interface and if it is one of the above ranges, you should remove
the 'norfc1918' option from the external interface's entry in
/etc/shorewall/interfaces.
You will want to assign your local addresses from one sub-network or
subnet and your DMZ addresses from another subnet. For our purposes, we
can consider a subnet to consists of a range of addresses x.y.z.0 -
x.y.z.255. Such a subnet will have a Subnet Mask of 255.255.255.0. The
address x.y.z.0 is reserved as the Subnet Address and x.y.z.255 is
reserved as the Subnet Broadcast Address. In Shorewall, a subnet is
described using Classless InterDomain Routing (CIDR) notation with
consists of the subnet address followed by "/24". The "24" refers to the
number of consecutive "1" bits from the left of the subnet mask.
Example sub-network:
Range: 10.10.10.0 - 10.10.10.255
Subnet Address: 10.10.10.0
Broadcast Address: 10.10.10.255
CIDR Notation: 10.10.10.0/24
It is conventional to assign the internal interface either the first
usable address in the subnet (10.10.10.1 in the above example) or the last
usable address (10.10.10.254).
One of the purposes of subnetting is to allow all computers in the subnet
to understand which other computers can be communicated with directly. To
communicate with systems outside of the subnetwork, systems send packets
through a gateway (router).
Your local computers (Local Computers 1 & 2) should be configured
with their default gateway set to the IP address of the firewall's
internal interface and your DMZ computers ( DMZ Computers 1 & 2) should be
configured with their default gateway set to the IP address of the
firewall's DMZ interface.
The foregoing short discussion barely scratches the surface regarding
subnetting and routing. If you are interested in learning more about IP
addressing and routing, I highly recommend "IP Fundamentals: What Everyone
Needs to Know about Addressing & Routing", Thomas A. Maufer,
Prentice-Hall, 1999, ISBN 0-13-975483-0.
The remainder of this quide will assume that you have configured your
network as shown here:
The default gateway for the DMZ computers would be 10.10.11.254 and the
default gateway for the Local computers would be 10.10.10.254.
WARNING: Your ISP might assign your external interface an RFC 1918
address. If that address is in the 10.10.10.0/24 subnet then you will need
to select a DIFFERENT RFC 1918 subnet for your local network and if it is
in the 10.10.11.0/24 subnet then you will need to select a different RFC
1918 subnet for your DMZ.
IP Masquerading (SNAT)
The addresses reserved by RFC 1918 are sometimes referred to as
non-routable because the Internet backbone routers don't forward packets
which have an RFC-1918 destination address. When one of your local systems
(let's assume local computer 1) sends a connection request to an internet
host, the firewall must perform Network Address Translation (NAT). The
firewall rewrites the source address in the packet to be the address of
the firewall's external interface; in other words, the firewall makes it
look as if the firewall itself is initiating the connection. This is
necessary so that the destination host will be able to route return
packets back to the firewall (remember that packets whose destination
address is reserved by RFC 1918 can't be routed accross the internet).
When the firewall receives a return packet, it rewrites the destination
address back to 10.10.10.1 and forwards the packet on to local computer 1.
On Linux systems, the above process is often referred to as IP
Masquerading and you will also see the term Source Network Address
Translation (SNAT) used. Shorewall follows the convention used with
Netfilter:
Masquerade describes the case where you let your firewall system
automatically detect the external interface address.
SNAT refers to the case when you explicitly specify the source address
that you want outbound packets from your local network to use.
In Shorewall, both Masquerading and SNAT are configured with entries in
the /etc/shorewall/masq file.
If your external firewall interface is eth0, your local interface
eth1 and your DMZ interface is eth2 then you do not need to modify the
file provided with the sample. Otherwise, edit /etc/shorewall/masq and
change it to match your configuration.
If your external IP is static, you can enter it in the third column
in the /etc/shorewall/masq entry if you like although your firewall will
work fine if you leave that column empty. Entering your static IP in
column 3 makes
processing outgoing packets a little more efficient.
If you are using the Debian package, please check your shorewall.conf
file to ensure that the following are set correctly; if they are not,
change them appropriately:
NAT_ENABLED=Yes (Shorewall versions earlier than 1.4.6)
IP_FORWARDING=On
Port Forwarding (DNAT)
One of your goals will be to run one or more servers on your DMZ
computers. Because these computers have RFC-1918 addresses, it is not
possible for clients on the internet to connect directly to them. It is
rather necessary for those clients to address their connection requests to
your firewall who rewrites the destination address to the address of your
server and forwards the packet to that server. When your server responds,
the firewall automatically performs SNAT to rewrite the source address in
the response.
The above process is called Port Forwarding or Destination Network Address
Translation (DNAT). You configure port forwarding using DNAT rules in the
/etc/shorewall/rules file.
The general form of a simple port forwarding rule in /etc/shorewall/rules
is:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
DNAT net dmz: [:]
If you don't specify the , it is assumed to be the same as
.
Example - you run a Web Server on DMZ 2 and you want to forward incoming
TCP port 80 to that system:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
DNAT net dmz:10.10.11.2 tcp 80 # Forward port 80 from the internet
ACCEPT loc dmz:10.10.11.2 tcp 80 #Allow connections from the local
network
A couple of important points to keep in mind:
When you are connecting to your server from your local systems, you must
use the server's internal IP address (10.10.11.2).
Many ISPs block incoming connection requests to port 80. If you have
problems connecting to your web server, try the following rule and try
connecting to port 5000 (e.g., connect to http://w.x.y.z:5000 where
w.x.y.z is your external IP).
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
DNAT net dmz:10.10.11.2:80 tcp 5000
If you want to be able to access your server from the local network using
your external address, then if you have a static external IP you can
replace the loc->dmz rule above with:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
DNAT net dmz:10.10.11.2:80 tcp 80 -
If you have a dynamic ip then you must ensure that your external interface
is up before starting Shorewall and you must take steps as follows (assume
that your external interface is eth0):
Include the following in /etc/shorewall/params:
ETH0_IP=`find_interface_address eth0`
Make your loc->dmz rule:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
DNAT loc
dmz:10.10.11.2:80 tcp 80 - $ETH0_IP
If you want to access your server from the DMZ using your external IP
address, see FAQ 2a.
At this point, add the DNAT and ACCEPT rules for your servers.
Domain Name Server (DNS)
Normally, when you connect to your ISP, as part of getting an IP address
your firewall's Domain Name Service (DNS) resolver will be automatically
configured (e.g., the /etc/resolv.conf file will be written).
Alternatively, your ISP may have given you the IP address of a pair of DNS
name servers for you to manually configure as your primary and secondary
name servers. It is your responsibility to configure the resolver in your
internal systems. You can take one of two approaches:
You can configure your internal systems to use your ISP's name servers. If
you ISP gave you the addresses of their servers or if those addresses are
available on their web site, you can configure your internal systems to
use those addresses. If that information isn't available, look in
/etc/resolv.conf on your firewall system -- the name servers are given in
"nameserver" records in that file.
You can configure a Caching Name Server on your firewall or in your
DMZ. Red Hat has an RPM for a caching name server (which also requires the
'bind' RPM) and for Bering users, there is dnscache.lrp. If you take this
approach, you configure your internal systems to use the caching name
server as their primary (and only) name server. You use the internal IP
address of the firewall (10.10.10.254 in the example above) for the name
server address if you choose to run the name server on your firewall. To
allow your local systems to talk to your caching name server, you must
open port 53 (both UDP and TCP) from the local network to the server; you
do that by adding the rules in /etc/shorewall/rules.
If you run the name server on the firewall: ACTION SOURCE DESTINATION
PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT loc fw tcp 53
ACCEPT loc fw udp 53
ACCEPT dmz fw tcp 53
ACCEPT dmz fw udp 53
Run name server on DMZ computer 1
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT loc dmz:10.10.11.1 tcp 53
ACCEPT loc dmz:10.10.11.1 udp 53
ACCEPT fw dmz:10.10.10.1 tcp 53
ACCEPT fw dmz:10.10.10.1 udp 53
Other Connections
The three-interface sample includes the following rules:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT fw net udp 53
ACCEPT fw net tcp 53
Those rules allow DNS access from your firewall and may be removed if you
commented out the line in /etc/shorewall/policy allowing all connections
from the firewall to the internet.
The sample also includes:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT loc fw tcp 22
ACCEPT loc dmz tcp 22
That rule allows you to run an SSH server on your firewall and in each of
your DMZ systems and to connect to those servers from your local systems.
If you wish to enable other connections between your systems, the general
format is:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT
Example - You want to run a publicly-available DNS server on your firewall
system:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT net fw tcp 53 #Allow DNS access from the internet
ACCEPT net fw udp
53 #Allow DNS access from the internet
Those two rules would of course be in addition to the rules listed above
under "If you run the name server on your firewall".
If you don't know what port and protocol a particular application uses,
look here.
Important: I don't recommend enabling telnet to/from the internet because
it uses clear text (even for login!). If you want shell access to your
firewall from the internet, use SSH:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT net fw tcp 22
Bering users will want to add the following two rules to be
compatible with Jacques's Shorewall configuration.
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT loc
fw udp
53
#Allow DNS Cache to work
ACCEPT loc fw tcp 80 #Allow weblet to work
Now modify /etc/shorewall/rules to add or remove other connections as
required.
Starting and Stopping Your Firewall
The installation procedure configures your system to start Shorewall
at system boot but beginning with Shorewall version 1.3.9 startup is
disabled so that your system won't try to start Shorewall before
configuration is complete. Once you have completed configuration of your
firewall, you can enable Shorewall startup by removing the file
/etc/shorewall/startup_disabled.
IMPORTANT: Users of the .deb package must edit /etc/default/shorewall and
set 'startup=1'.
The firewall is started using the "shorewall start" command and stopped
using "shorewall stop". When the firewall is stopped, routing is enabled
on those hosts that have an entry in /etc/shorewall/routestopped. A
running firewall may be restarted using the "shorewall restart" command.
If you want to totally remove any trace of Shorewall from your Netfilter
configuration, use "shorewall clear".
The three-interface sample assumes that you want to enable routing
to/from eth1 (your local network) and eth2 (DMZ) when Shorewall is
stopped. If these two interfaces don't connect to your local network and
DMZ or if you want to enable a different set of hosts, modify
/etc/shorewall/routestopped accordingly.
WARNING: If you are connected to your firewall from the internet, do not
issue a "shorewall stop" command unless you have added an entry for the IP
address that you are connected from to /etc/shorewall/routestopped. Also,
I don't recommend using "shorewall restart"; it is better to create an
alternate configuration and test it using the "shorewall try" command.
Additional Recommended Reading
I highly recommend that you review the Common Configuration File Features
page -- it contains helpful tips about Shorewall features than make
administering your firewall easier.
Last updated 11/15/2003 - Tom Eastep
Copyright 2002, 2003 Thomas M. Eastep
Documentation Index
